When creating cyclic patterns in masks or reticles for the production of for instance displays, such as TFT-LCD's or plasma-displays, or when creating a pattern directly on a semiconducting wafer, i.e., direct writing, a key quality requirement is the absence of defects, such as shade differences, light and dark fields, stripes or lines in said pattern.
Deviations, e.g. CD (critical dimension) or positioning errors, causing said defects are usually very small, from a few hundred nanometers down to or below single nanometers. Deviations of that size spread over a relatively large area on a substrate, such as a display photo mask, which may be 1500×1200 mm, or a semiconducting wafer may be very difficult, for not saying impossible, to detect by measuring. Unfortunately a human eye is very sensitive to systematic changes and therefore may be able to detect such small deviations as stripes in the image. The human eye is extremely sensitive to periodical intensity variations in the image. The viewing distance will influence the appearance of said periodical intensity variations. Generally speaking, periodical intensity variations may be detectable by the human eye if the difference in contrast is around and above 0,5% in a spatial frequency range of 1-20 mm. For normal viewing distances periodical intensity variations below about 1 mm may not show up.
Periodical defects may be caused by the beating frequencies between a pattern pitch and a system pitch in a certain direction. The pattern pitch may be defined as the distance between equal features in the pattern. The pattern pitch may be different in an X and a Y direction of said pattern. One system pitch in a raster scanning system is a Y-pitch, defining a distance between two adjacent center of gravity of exposure spot sizes along a sweep direction of said exposure beam. A numerous exposure spots, in the range of several hundreds, along the sweep direction are forming a scan line on the workpiece. The exposure spot may preferably be continuously on within a scan line, but may be switched off at any given time determined by a frequency of a clock generator connectable to a modulator which is modulating said exposure beam according to desired pattern data. Another system pitch is an X-pitch, defining a distance between two adjacent parallel scan lines of said exposure beam. A numerous scan lines in X-direction may form a strip. Strips stitched together will form a desired pattern on the workpiece.
Unfortunately, masks, reticles or semiconducting wafers patterned by using multiple exposure beams for increasing the writing speed may present certain side effects such as increased CD (Critical Dimension)-error. i.e., lines or features printed on a workpiece have less uniform line widths and/or features with edge roughness.
What is needed is a method and apparatus, which is capable to pattern any kind of pattern without increasing CD-error and/or feature roughness when creating a pattern on the workpiece by using multiple exposure beams.